1. Field of the Invention
The present invention relates to a method for generating an NC (numerical control) program which is used when an NC machine equipped with an end mill serving as a cutting tool cuts into a complex form such as a mold, and which reduces wear and damage to the cutting tool and thereby permits efficient cutting, and an apparatus, and a computer memory and program products for implementing the above-mentioned method.
2. Description of the Related Art
Molds for molding various industrial products are fabricated generally by machining of metal materials by using an NC machine equipped with an end mill serving as a cutting tool. In fabrication of such a mold, shape design is first carried out using a CAD (computer aided design) system, and then machining process design is carried out using a CAM (computer aided manufacturing) system. On the basis of this process design, machining is carried out using an NC machine.
Nevertheless, in the above-mentioned process design carried out using the CAM system on the basis of the shape data given from the CAD system, the resultant data contains merely geometry-related data for the machining processes such as: the selection of cutting tools (end mills) to be used; and the tool paths for the selected cutting tools. Thus, in order that the NC machine can actually perform the machining, an NC program needs to be generated on the basis of the result of the process design carried out by the CAM system.
This NC program sets machining conditions in each portion along the tool paths for the cutting tools determined as described above. Generally in the prior art, the NC program is generated manually by an operator on the basis of the operator""s experience. In an exemplary case where an end mill is used as a cutting tool, the major cutting condition to be set is the feed rate along the tool path, or more specifically, the amount of feed per tooth (mm/tooth) for the teeth arranged along the periphery of the end mill.
In the generation of the NC program, it is desired to reduce wear in the cutting tools, avoid damage such as chipping to the cutting tools, and thereby extend the life of the cutting tools. At the same time, it is desired to reduce machining time as much as possible. An NC program satisfying these contradictory requirements is laborious to generate even for an experienced operator. Further, in generation of an NC program used for machining of a mold having a complex shape to be cut, it is substantially impossible to optimize the feed rate along the entire tool paths.
As such, in generation of an NC program in the prior art, the reduction of wear and the avoidance of damage to the cutting tools have been prioritized, while the machining conditions, such as feed rate, have been set lower with a margin for safety. This has caused the problem that the machining time is not reduced satisfactorily.
In order to resolve such a problem, NC machines have been developed in which during the machining according to an NC program generated in advance, the actual load exerted on the cutting tool is detected, whereby the numerical control commands corresponding to the machining conditions are corrected in real time by feedback based on the result of the detection. For example, Reissued PCT International Publication No. WO98/19822 discloses an NC machine in which the actual load exerted on the cutting tool is detected, whereby a simulation of the machining situation at future machining positions is carried out on the basis of the result of the detection, and whereby the numerical control commands corresponding to the machining conditions are corrected by feedforward based on the result of the simulation.
Nevertheless, in the former apparatuses carrying out feedback control based on the result of detection of the load on the cutting tool, there is the problem that a sudden load change can cause a delay in the machining condition correction. When the control gain is increased in order to reduce the delay, another problem can occur that an excessive correction is carried out for the sudden load change. As such, there has been a difficulty in setting of the control gain to an appropriate value.
On the other hand, in the latter apparatus carrying out feedforward control based on the result of detection of the load on the cutting tool, there is the problem that when the simulation is carried out on the basis of an incidental external disturbance, such as chip jamming around the cutting tool and local variation in the material characteristics, inappropriate correction can be carried out. When the control gain is reduced in order to reduce such a problem, another problem can occur that the normal feedforward correction of the machining conditions delays for ordinary load changes. As such, there has been a difficulty in setting of the control gain to an appropriate value.
In order to compensate the above-mentioned problems in the feedback system and the feedforward system, both systems may be used in combination. Nevertheless, such a combination results in a complex control system, and thereby causing an enhanced difficulty in setting of the control gains to appropriate values.
The invention has been devised in order to resolve the above-mentioned problems. An object of the invention is to provide an NC program generating method and apparatus in which when tool paths are determined in a process design using a CAM system, machining conditions in each portion along the tool paths are determined in advance by a simple method, whereby an NC program containing the machining conditions is generated automatically. Another object of the invention is to provide computer memory and program products for implementing the above-mentioned method.
An aspect of the invention is a method for generating a program used for NC machining in which an end mill having a plurality of teeth arranged along the periphery thereof and rotating around the axis thereof is fed along a tool path defined on a machined surface for machining the machined surface into a predetermined shape, the method comprising the steps of calculating, in case of a ball end mill, a maximum undeformed chip thickness, a cutting arc length or an engage angle, and an active angle, or, in case of a straight end mill, the maximum undeformed chip thickness, the cutting arc length or the engage angle, and an axial depth of cut for each tooth of the end mill in each portion along the tool path by using the size of an end mill to be used for machining, the cutting depth in the radial direction of the end mill, and the amount of feed per tooth; obtaining an estimated value of the cutting force in each portion along the tool path by applying the result of the calculation to a response surface whose explanation variables are composed of the maximum undeformed chip thickness, the cutting arc length or the engage angle, and the active angle or the axial depth of cut; and determining the feed rate of the end mill in each portion along the tool path to maintain the estimated value of the cutting force at an appropriate value.
Another aspect of the invention is an apparatus for generating a program used for NC machining in which an end mill having a plurality of teeth arranged along the periphery thereof and rotating around the axis thereof is fed along a tool path defined on a machined surface for machining the machined surface into a predetermined shape, the apparatus comprising: calculating means for calculating, in case of a ball end mill, a maximum undeformed chip thickness, a cutting arc length or an engage angle, and an active angle, or, in case of a straight end mill, the maximum undeformed chip thickness, the cutting arc length or the engage angle, and an axial depth of cut for each tooth of the end mill in each portion along the tool path by using the size of an end mill to be used for machining, the cutting depth in the radial direction of the end mill, and the amount of feed per tooth; obtaining means for obtaining an estimated value of the cutting force in each portion along the tool path by applying the result of the calculating means to a response surface whose explanation variables are composed of the maximum undeformed chip thickness, the cutting arc length or the engage angle, and the active angle or the axial depth of cut; and means for determining the feed rate of the end mill in each portion along the tool path to maintain the estimated value of the cutting force obtained by the obtaining means at an appropriate value.
Further another aspect of the invention is a computer memory product readable by a computer to execute a method for generating a program used for NC machining in which an end mill having a plurality of teeth arranged along the periphery thereof and rotating around the axis thereof is fed along a tool path defined on a machined surface for machining the machined surface into a predetermined shape, the computer memory product comprising: calculating, in case of a ball end mill, a maximum undeformed chip thickness, a cutting arc length or an engage angle, and an active angle, or, in case of a straight end mill, the maximum undeformed chip thickness, the cutting arc length or the engage angle, and an axial depth of cut for each tooth of the end mill in each portion along the tool path by using the size of an end mill to be used for machining, the cutting depth in the radial direction of the end mill, and the amount of feed per tooth; obtaining an estimated value of the cutting force in each portion along the tool path by applying the result of the calculation to a response surface whose explanation variables are composed of the maximum undeformed chip thickness, the cutting arc length or the engage angle, and the active angle or the axial depth of cut; and determining the feed rate of the end mill in each portion along the tool path to maintain the estimated value of the cutting force at an appropriate value.
Further another aspect of the invention is a computer program product for use with NC machine, comprising: a computer usable storage medium having a computer readable program code embodied therein for generating a program used for NC machining in which an end mill having a plurality of teeth arranged along the periphery thereof and rotating around the axis thereof is fed along a tool path defined on a machined surface for machining the machined surface into a predetermined shape, the computer readable program code comprising: causing a computer to calculate, in case of a ball end mill, a maximum undeformed chip thickness, a cutting arc length or an engage angle, and an active angle, or, in case of a straight end mill, the maximum undeformed chip thickness, the cutting arc length or the engage angle, and an axial depth of cut for each tooth of the end mill in each portion along the tool path by using the size of an end mill to be used for machining, the cutting depth in the radial direction of the end mill, and the amount of feed per tooth; causing a computer to obtain an estimated value of the cutting force in each portion along the tool path by applying the result of the calculation to a response surface whose explanation variables are composed of the maximum undeformed chip thickness, the cutting arc length or the engage angle, and the active angle or the axial depth of cut; and causing a computer to determine the feed rate of the end mill in each portion along the tool path to maintain the estimated value of the cutting force at an appropriate value.
In the invention, in each portion along the end-mill tool paths determined in the shape design using a CAD system and the process design using a CAM system, a maximum undeformed chip thickness, a cutting arc length or an engage angle, and an active angle for each tooth of the end mill are calculated when a ball end mill is used in machining, or the maximum undeformed chip thickness, the cutting arc length or the engage angle, and an axial depth of cut for each tooth of the end mill are calculated when a straight end mill is used in machining, on the basis of the geometrical relation with the corresponding machined surface. The result of the calculation is applied to a response surface corresponding to the explanation variables composed of these values, whereby an estimated value of the cutting force exerted on the end mill is obtained accuracy. Then, the feed rate incorporated in an equation expressing the maximum undeformed chip thickness is determined such that the estimated value of the cutting force is maintained at a predetermined appropriate value at each position. As described above, the feed rate is defined as the amount of feed per tooth (mm/tooth) for the teeth arranged along the periphery of the end mill.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.